ANOMALOUS DIFFRACTION EFFECTS IN THE SIZING OF SOLID PARTICLES IN LIQUIDS

A study has been made of anomalous diffraction effects in the sizing of solid particles in liquids using a Malvern laser diffraction sizer. The particles were N.B.S. standard reference material 1004 glass spheres. The five liquids used were chosen to give a range of refractive index (R.I.) above and below that of the glass; the ratio of solid R.I. to liquid R.I. (m) ranged from 0.92 to 1.15. Measurements were made of the change in the light energy distribution pattern as the value of \m - 1\ was increased or reduced towards zero. At high values of \m - 1\ the measured light energy distribution pattern was predicted adequately by the Fraunhofer diffraction approximation to Mie theory and the Fraunhofer computer software could be used to derive particle size distribution from the light energy pattern. For values of \m - 1\ near to zero, the Fraunhofer approach was in error but the anomalous diffraction approximation and software proved adequate for the back-calculation of size distribution from energy pattern.